The rubber and plastic industries use screen-type straining devices to separate the oversize material from the extrusion, the restrictive holes of which progressively become plugged, causing an increase in the extrusion pressure required to process the material. The progressive change in pressure requires additional input work and therefore results in uneven processing. The plastic industry has advanced to the stage where continuous screen-changing devices are used to improve uniformity.
Extrusion mixing of rubber-like materials is limited by heat build-up in the process material, and therefore any improvement in mixing has to be accomplished without any increase in input power. Thus, the length of a rubber extruder operating with a full screw is limited to approximately seven diameters for process material as used in tire manufacturing.